String Comparison Mistakes

In Java, Strings are objects, but their widespread use has led to special compiler optimizations, most notably the String Pool. This optimization often confuses developers about the difference between reference checks and value checks.

The String Pool Trap

When you write String s1 = "hello";, the JVM checks the String Pool. If "hello" is present, it returns the reference. If not, it creates it in the pool. Therefore, two literal-assigned strings point to the same address and s1 == s2 is true.

However, writing String s3 = new String("hello"); forces the creation of a new object on the heap, bypassing the pool checks. Now, s1 == s3 is false, even though their content is identical.

Code Walkthrough

This class demonstrates String references, literal pooling, and safe null checks.

public class StringPitfallsDemo {
    public static void main(String[] args) {
        String s1 = "java";
        String s2 = "java";
        String s3 = new String("java");
// Literal pooling optimization
        System.out.println(s1 == s2);      // true (same address in String Pool)
        System.out.println(s1 == s3);      // false (different heap memory addresses)
        System.out.println(s1.equals(s3)); // true (same character content)
// The intern() method manually returns pool reference
        String s4 = s3.intern();
        System.out.println(s1 == s4);      // true (intern returns the pooled instance)
// Dangers of NullPointerExceptions (NPE)
        String input = null;
        try {
            // Bad Practice: calling equals() on a variable that might be null
            boolean check = input.equals("admin"); 
        } catch (NullPointerException e) {
            System.out.println("NPE caught! Avoid calling equals on variables.");
        }
// Good Practice 1: literal-first comparison
        boolean safeCheck1 = "admin".equals(input); // false, no NPE thrown
// Good Practice 2: Utility checks
        boolean safeCheck2 = java.util.Objects.equals(input, "admin"); // false, safe
    }
}

Interview-Relevant Information